Cyclohexylbenzene is a product of increasing importance in the chemical industry since it offers an alternative route to the Hock process for the production of phenol. The Hock process is a three-step process in which benzene is alkylated with propylene to produce cumene, the cumene is oxidized to the corresponding hydroperoxide and then the hydroperoxide is cleaved to produce equimolar amounts of phenol and acetone.
Oxidation of cyclohexylbenzene has potential as an alternative route for the production of phenol since it co-produces cyclohexanone, which has a growing market and is used as an industrial solvent, as an activator in oxidation reactions and in the production of adipic acid and cyclohexanone resins. However, this alternative route requires the development of a commercially viable process for producing the cyclohexylbenzene precursor.
It has been known for many years that cyclohexylbenzene can be produced from benzene either directly by alkylation with cyclohexene or by the process of hydroalkylation or reductive alkylation. In the latter process, benzene is heated with hydrogen in the presence of a catalyst such that the benzene undergoes partial hydrogenation to produce cyclohexene which then alkylates the benzene starting material. Thus, U.S. Pat. Nos. 4,094,918 and 4,177,165 disclose hydroalkylation of aromatic hydrocarbons over catalysts which comprise nickel- and rare earth-treated zeolites and a palladium promoter. Similarly, U.S. Pat. Nos. 4,122,125 and 4,206,082 disclose the use of ruthenium and nickel compounds supported on rare earth-treated zeolites as aromatic hydroalkylation catalysts. The zeolites employed in these prior art processes are zeolites X and Y. In addition, U.S. Pat. No. 5,053,571 proposes the use of ruthenium and nickel supported on zeolite beta as the aromatic hydroalkylation catalyst. However, these earlier proposals for the hydroalkylation of benzene suffered from the problems that the selectivity to cyclohexylbenzene was low particularly at economically viable benzene conversion rates and large quantities of unwanted by-products were produced.
More recently, U.S. Pat. No. 6,037,513 has disclosed that cyclohexylbenzene selectivity in the hydroalkylation of benzene can be improved by contacting the benzene and hydrogen with a bifunctional catalyst comprising at least one hydrogenation metal and a molecular sieve of the MCM-22 family. The hydrogenation metal is preferably selected from palladium, ruthenium, nickel, cobalt and mixtures thereof and the contacting is conducted at a temperature of about 50 to 350° C., a pressure of about 100 to 7000 kPa, a benzene to hydrogen molar ratio of about 0.01 to 100 and a WHSV of about 0.01 to 100. The '513 patent discloses that the resultant cyclohexylbenzene can then be oxidized to the corresponding hydroperoxide and the hydroperoxide decomposed to the desired phenol and cyclohexanone.
However, although the use of MCM-22 family catalysts has significantly increased product selectivity, the manufacture of cyclohexylbenzene both by direct alkylation and by benzene hydroalkylation still tends to be accompanied by the co-production of significant quantities of by-products. One of the more prevalent contaminants is polycyclohexylbenzenes, which typically comprise up to 20 wt % of the conversion products. Thus, for the overall process to be economically feasible, it is necessary to convert these polycyclohexylbenzenes into additional useful monocyclohexylbenzene product.
One possible method of converting polycyclohexylbenzenes into additional monocyclohexylbenzene is by transalkylation with additional benzene, a solution which is addressed in the '513 patent by effecting the transalkylation in the presence of a catalyst containing the same molecular sieve as used in the hydroalkylation catalyst, namely an MCM-22 family catalyst, but in the absence of the metal components on the hydroalkylation catalyst and in the absence of a hydrogen co-feed. Other transalkylation processes are described in U.S. Pat. No. 6,489,529 and our co-pending PCT Application Nos. PCT/EP2008/006072 and PCT/US2010/031029.
Another process for producing additional cyclohexylbenzene from by-product polycyclohexylbenzenes is described in our co-pending PCT Application No. PCT/2011/023537 and comprises dealkylation of the polycyclohexylbenzenes in the presence of an acid catalyst, such as at least one aluminosilicate, aluminophosphate, or silicoaluminphosphate.
The above methods of converting polycyclohexylbenzenes into additional useful monocyclohexylbenzene product require initial separation of the polycyclohexylbenzenes from the remainder of the alkylation or hydroalkylation process effluent. This is generally effected by a multi-stage fractionation process, in which unreacted benzene and cyclohexylbenzene product are removed from the process effluent in sequential fractionation stages leaving a C12+fraction containing the polycyclohexylbenzenes. Optionally, the C12+ fraction is further fractionated to purge a heavies stream from the polycyclohexylbenzenes. Currently, in order to achieve satisfactory separation, each fractionation stage must be operated under vacuum and at a relatively high temperature. Not only is such operation expensive but, in a commercial setting, vacuum operation is likely to result in air ingress and hence formation of oxygenated hydrocarbons. Not only does this lead to loss of valuable product but also the oxygenated hydrocarbons may deactivate the catalyst employed in the downstream transalkylation or dealkylation of the polycyclohexylbenzenes.
According to the invention, it has now been found that separation of cyclohexylbenzene from polycyclohexylbenzenes in the effluent from the reaction of benzene with cyclohexene can be facilitated by the injection of one or more C1 to C11 hydrocarbons or hydrogen, or C4 to C6 hydrocarbons, such as in vapor phase, into the separation device (e.g., fractionation unit). This step allows the fractionation unit to be operated at or near atmospheric pressure and a moderate bottoms temperature, typically from about 190° C. to about 300° C., especially from about 190° C. to about 241° C.